Effect of Light Intensity and Temperature on Photosynthetic Rate, Leaf Starch Content and Ultrastructure of Paspalum dilatatum

Abstract

;Paspalum dilatatum plants were grown in growth cabinets under two light intensities (224 and 90 W m-2) at day/night temperatures of 27.5/20.0, 20.0/12.5, 17.5/12.5 and 15.0/10.0°C. Additional plants were grown in the glasshouse and outside. Relative growth rate was closely correlated with the mean hourly photosynthetic rate of the plant during the day. Starch varied from 0.8 to 5.3 % of the leaf dry weight at the end of the night and from 3.0 to 11.6% at the end of the day. Photosynthetic rate of cabinet-grown plants remained constant during the day as starch accumulated. The rate declined for plants from outside, although their leaf starch levels were no higher than those of cabinet-grown plants. No evidence was found of feedback repression of photosynthetic rate caused by assimilate accumulation. There was only minor variation in mesophyll chloroplast structure between plants grown at different light intensities at the same temperature. A decline in temperature below 20.0/12.5°C increased swelling of mesophyll chloroplasts, as well as reducing development of the peripheral reticulum. The structure of bundle sheath chloroplasts was little affected by the environment apart from its effect on starch accumulation. While growth rate increased rapidly as temperature increased, there was much less effect of temperature on accumulation of starch during the day. It was concluded that photosynthesis and the accumulation of starch are less sensitive to temperature than is overall plant growth.